Just about every single circuit on this car is chassis grounded...there are very few dedicated ground wires.
This means you need to make sure the dash is grounded to the unibody is grounded to the subframe is grounded to the engine . And the radiator support must also be grounded to the subframe. And then you need a big, fat, nasty ground cable going back to the battery. There simply MUST be electrical continuity between ALL of these major parts of the car. Only THEN can you ground every circuit to any of these major metal items and expect it to work properly.
When ONE of these grounds is poor, things start going haywire. Lights that are supposed to be on, glow dim. Lights that are supposed to be off, glow randomly. Starters crank slowly when engine is hot, and some things simply stop working. I betcha dollars to donuts you have a ground somewhere that is missing/corroded/painted/disconnected/undersized. And the bad ground may or may not be near your dash cluster or wiper switch/motor ground. But it very well might...and I'd still recommend you look there first.
With these cars, we need to recognize that the painted dash is bolted to the painted unibody (poor ground), and the subframe is connected to the unibody with rubber mounts (poor/no ground), and the radiator support is connected to the subframe with rubber mounts (poor/no ground), and the engine is connected to the subframe with rubber mounts (poor/no ground). And your battery is grounded usually only to the engine itself (good ground...yay!...assuming it's clean/corrosion and PAINT free/heavy gauge).
Because of the above vehicle construction methods; copper straps, wires, and cables MUST connect all of these components to complete circuits back to the battery...the painted surfaces and rubber connection points just can't effectively carry electricity...and when that electricity can't find an easy/unintertupted path back to the battery, it finds alternate paths, like maybe right thru light bulbs that are supposed to be OFF, causing them to glow at varying brightnesses, etc.
You need to identify all of these major grounds and inspect them. And if you can't find one of the above mentioned ground paths, just easily create one, by adding a ground strap or cable somewhere hidden...a nice copper connection point, heavy gauge, to metal ground bare and shiny.
There is no more poorly grounded car than a just-completed rotisserie restoration where someone has spent ridiculous time and effort to paint and coat EVERY SINGLE SURFACE of the car and engine, but then neglected to grind that pretty paint completely OFF of the connection points for every single ground. As painful as that sounds to some to grind 50 to 100 locations of paint OFF of a freshly painted car during reassembly, it absolutely must be carefully done at every single electrical path, or things won't work right.
There is no such thing as too many ground paths on these cars. And it is often the prettiest and most over-restored cars that have the fewest/worst ground paths.
